Our board of directors has adopted and the shareholders have approved two stock option award plans. The 2010 Thermon Group Holdings, Inc. Restricted Stock and Stock Option Plan (“2010 Plan”) was approved on July 28, 2010. The 2010 Plan authorized the issuance of 2,767,171 stock options or restricted shares (on a post-stock split basis). On April 8, 2011, the board of directors approved the Thermon Group Holdings, Inc. 2011 Long-Term Incentive Plan (“2011 LTIP”). The 2011 LTIP made available 2,893,341 shares of the Company’s common stock that may be awarded to employees, directors or non-employee contractors as compensation in the form of stock options, restricted stock awards or restricted stock units.

At September 30, 2018, there were 344,487 options outstanding. For the three months ended September 30, 2018 and 2017, stock compensation expense was $1,085 and $947, respectively, and $2,089 and $1,732 for the six months ended September 30, 2018 and 2017, respectively.

During the six months ended September 30, 2018, 89,114 restricted stock units were issued to our employees with an aggregate grant date fair value as determined by the closing price of our stock on the respective grant dates of $2,098. The awards will be expensed on a straight-line basis over the three-year service period. At each anniversary of the restricted stock units' grant date, a proportionate number of stock units will become vested for the employees and the shares will become issued and outstanding. Additionally, during the six months ended September 30, 2018, we made a one-time grant to our named executive officers of 10,613 restricted stock units which will cliff vest on March 31, 2020. The total grant date fair value, as determined by the closing price of our common stock on the date of grant, was $250. The expense will be recognized ratably over the vesting period.

We maintain a plan to issue our directors awards of fully vested common stock every three months for a total award over a twelve-month period of approximately $420. During the three and six months ended September 30, 2018, 5,240 and 10,452 fully vested common shares, respectively, were issued in the aggregate to our directors. The aggregate grant date fair value as determined by the closing price of our common stock on the grant date was $120 and $235 for the three and six months ended September 30, 2018, respectively. The fair value of the awards is expensed on each grant date.

During the six months ended September 30, 2018, a target amount of 11,533 performance stock units were issued to certain members of our senior management that had a total grant date fair value of $320. The performance indicator for these performance stock units is based on the market performance of our stock price, from the date of grant through March 31, 2021, relative to the market price performance of a pre-determined peer group of companies. Since the performance indicator is market-based, we used a Monte-Carlo valuation model to calculate the probable outcome of the performance measure to arrive at the fair value. The requisite service period required to earn the awards is through March 31, 2021. We will expense the fair value of the performance stock units over the service period on a straight-line basis whether or not the stock price performance condition is met. At the end of the performance period, the performance stock units will be evaluated with the requisite number of shares being issued. The possible number of shares that could be issued ranges from zero to 23,066 in the aggregate. Shares that are not awarded at the measurement date will be forfeited.

In addition to the market-based performance stock units issued to certain members of senior management, we also granted these individuals, during the six months ended September 30, 2018, a target amount of 46,032 performance stock units based on the Company's Adjusted EBITDA performance over a three-year period ending March 31, 2021. The total grant date fair value, as determined by the closing price of our common stock on the date of the grant, was $1,084. At each reporting period, we will estimate how many awards senior management may achieve and adjust our stock compensation expense accordingly. At the end of the performance period, the performance stock units will be evaluated with the requisite number of shares issued. The possible number of shares that could be issued under such performance stock units ranges from zero to 92,064 in the aggregate. Shares that are not awarded after the end of the measurement period will be forfeited. Additionally, during the six months ended September 30, 2018, we made a one-time grant to our named executive officers of a target amount of 10,613 performance stock units based on the adjusted EBITDA contribution of the acquired THS operations for the period beginning on October 30, 2017, the closing date of the THS acquisition, and ending on March 31, 2020. The total grant date fair value, as determined by the closing price of our common stock on the date of grant, was $250. The expense will be recognized ratably over the vesting period. At the end of the performance period, the performance stock units will be evaluated with the requisite number of shares issued. The possible number of shares that could be issued under such performance stock units ranges from zero to 15,920 in the aggregate. Shares that are not awarded after the end of the performance period will be forfeited.